A Ukrainian official said three Russian tanks and several armoured vehicles crossed its eastern border on Thursday, even as Ukraine’s president made phone calls to the Russian and German leaders seeking support for his plan to end fighting in the region.

Ukraine’s interior minister said military forces that were in the area fighting pro-Moscow separatists attacked the convoy of armoured vehicles. He did not directly accuse Moscow of sending the tanks, but said it showed Russia had failed to fulfill promises to tighten border controls.

Russia has denied sending troops or weapons to Ukraine, describing Russian citizens who have joined the armed separatists as volunteers. There was no independent confirmation that the tanks had come from Russia.

In Washington, U.S. State Department spokeswoman Jen Psaki said if the military incursion was confirmed, it would be a "serious and disturbing escalation of the crisis in eastern Ukraine.”

The reported incursion followed statements earlier Thursday by Russia's foreign minister that the separatists were ready for a cease-fire but that Kiev had to initiate the process.

Blast rocks Donetsk

Late Thursday, an explosion shook the center of the major eastern city of Donetsk, where the rebels have taken over a regional administration building. An AP reporter nearby heard the explosion and arrived to see a van in flames in front of the building. He saw three injured people being taken away.

'We know that the rebels in the southeast are ready to hold fire, but the first step by all rights should be made by the Kyiv authorities.'- Sergey Lavrov, Russia's foreign minister

The breakaway Donetsk People's Republic said on its Twitter feed that the van was used by one of the group's leaders, Denis Pushilin, but said he was not in the vehicle. The same tweet said four people were injured and one was in grave condition.

Ukrainian President Petro Poroshenko, who took office less than a week ago, told Russian President Vladimir Putin that it was "unacceptable" that tanks had crossed the border, according to his spokesman, Svyatoslav Tsegolko. A Kremlin statement said Poroshenko told Putin about his plan for resolving the crisis in the east, but did not say whether they discussed the tanks.

The Ukrainian president also spoke Thursday with German Chancellor Angela Merkel, following a call the previous day with U.S. Vice President Joe Biden. Poroshenko has said he is willing to negotiate, but not with what he calls terrorists, and could offer amnesty to those who don't have "blood on their hands."

Armoured vehicles enter Ukraine

A van burns after an explosion in the centre of Donetsk, one of the eastern Ukrainian cities that has been racked by violence in recent months. (Evgeniy Maloletka/AP)

Interior Minister Arsen Avakov said a "column" with armored vehicles had crossed from Russia through border control points controlled by separatists near the village of Dyakove in eastern Ukraine. Three tanks went to the town of Snizhne, about 40 kilometres from Dyakove, and one remained there while the two others headed toward the town of Horlivka and were engaged by the Ukrainian military, he said. He added that part of the column was destroyed.

Avakov said the incursion had been going on for three days and took place despite Russian statements of interest in a peaceful solution and promises to increase control over the border.

U.S. Secretary of State John Kerry spoke Wednesday with Lavrov and urged him to encourage Putin to engage directly with Poroshenko, Psaki said.

"He also encouraged that conversation or engagement to focus on de-escalating the situation on the ground, and he called on Russia to halt the flow of militants and arms from Russia into eastern Ukraine, which is clearly relevant in this case," the spokeswoman said.

Ukrainian rebels ready to hold fire: Lavrov

Russia's U.N. ambassador said Thursday that he intends to introduce a UN Security Council resolution aimed at stopping the violence in Ukraine. Vitaly Churkin told reporters that it will focus on political efforts being carried out by the Organization for Security and Cooperation in Europe, "so far not successfully."

Russian Foreign Minister Sergey Lavrov said earlier Thursday the resolution would concentrate on demanding fulfillment of proposals in the OSCE "road map" to resolve the conflict. It calls for non-violence, disarmament, national dialogue and elections.

Lavrov said Russia was not seeking authorization to send in peacekeeping troops. The Ukrainian rebels have suggested that Russia should send peacekeepers, but Moscow says that could only be done with UN authorization.

"We know that the rebels in the southeast are ready to hold fire, but the first step by all rights should be made by the Kyiv authorities," Lavrov said.